Power and bandwidth are resources that are carefully conserved by digital transmission systems through the proper selection of modulation and error correction schemes. Quadrature Amplitude Modulation (QAM) is one form of multilevel amplitude and phase modulation that is frequently employed in digital communication. A QAM system modulates a source signal into an output waveform with varying amplitude and phase.
QAM is a form of two-dimensional symbol modulation composed of a quadrature (orthogonal) combination of two PAM (pulse amplitude modulated) signals. Data to be transmitted is mapped to a two-dimensional, four quadrant signal space, or constellation, having a plurality of signal (phasor) points each representing a possible transmission level. Each constellation signal point is commonly called a "symbol" and is defined by a unique binary code. The QAM constellation employs "I" and "Q" components to signify the in-phase and quadrature components, respectively, where a QAM data word or symbol is represented by both I and Q components.
QAM signals may employ words or symbols of different lengths and/or constellation shapes (e.g., circular, square, rectangular, cross, hexagonal or any arbitrary shapes). Although the present invention is described using a square QAM constellation, those skilled in the art will realize that the present invention can be modified for other QAM constellation shapes or QAM signal word of different lengths.
Generally, an increase in the number of phasor points (finer, higher order constellations) within the QAM constellation permits a QAM signal to carry more information, but the increase in density of the phasor points creates a disadvantage where the transmitted power is no longer constant. In fact, if the average transmitted signal power is limited, the maximum I and Q values are nearly the same for all the QAM levels, thereby causing the constellation points to be closely spaced as the QAM level increases. Since the distance between phasor points on a QAM constellation generally decreases with additional phasor points, it increases the complexity of distinguishing neighboring phasor points, and translates into a more expensive and complex receiver.
Thus, many older or existing DBS (Direct Broadcasting Satellite) systems employ Quaternary (Quadriphase) Phase Shift Keying (QPSK) modulation systems. For QPSK, a synchronous data stream is modulated onto a carrier frequency before transmission over the satellite channel, where the carrier can have four (phase) states, e.g., 45.degree., 135.degree., 225.degree. or 315.degree.. Thus, similar to QAM, QPSK employs quadriphase or quadrature modulation where the phasor points can be uniquely described using two orthogonal coordinate axes.
However, in contrast to QAM, QPSK has a "constant envelope", i.e., the pair of coordinate axes can be associated with a pair of quadrature carriers with a constant amplitude, thereby creating a four level constellation (four phasor points having a phase rotation of 90.degree.). This large separation of phasor points decreases the complexity of performing carrier recovery operation at a receiver, and the constant envelop is extremely desirable for power sensitive satellite transmission systems. Also, the constant power level allows the satellite amplifiers to operate in saturation.
Nevertheless, the desire to increase the data throughput over a satellite channel has created a strong incentive to consider other transmission schemes instead of QPSK. Due to the existence of numerous QPSK systems, it is herein recognized as desirable to provide a method and apparatus for processing a signal such as a QAM signal using a backward compatible hierarchical coding scheme to increase data transmission using finer constellations, while maintaining compatibility with existing systems including DBS systems.